Fluorescent polymer beads find application in myriad areas like multicolor emission, bar-coding, photonic crystals, self assembly study and as a standard in the fluorescence techniques like flow cytometry, cell sorting, sensing and imaging etc. Some of the important criteria for application of the fluorescent beads are monodispersity, non-leaching of the fluorophore, thermal and photo stability etc.
Fluorescent polymer particles are generally synthesized by incorporation of the dye with the polymer composite, physical adsorption, encapsulation of the dye in block copolymers by hydrophobic-hydrophilic interaction, gradual solvent evaporation, controlled mixing and so on. However these methods often face the problem of the dye leakage causing background fluorescence interference.
Synthesis of highly cross-linked polystyrene beads of 9.2 μm by seed polymerization with styrene as monomer and divinylbenzene as cross linker stained by gradual solvent evaporation method using dyes such as rhodamine 101 and acridine orange is reported in European Polymer Journal 45 (2009) 550-556 Qi Zhang, et al.
The fluorescent polymer particles can also be synthesized by post polymerization dispersion techniques. However, these systems require additional steps after polymerization and control over uniform emission and monodispersity of the polymer particle is also difficult.
To overcome these problems, copolymerization with polymerizable fluorophore and covalently attaching the fluorophore to the polymer chains is a good means to prevent leakage and obtain strongly fluorescent polymer particle with uniform distribution of the fluorophore in the polymer backbone. Preparation of polystyrene fluorescent microspheres by the dispersion copolymerization and absorption method is reported in J. Mater. Chem., 2009, 19, 2018-2025 by Qing-Hao Liu, et al., whereas Laser velocimetry technique to obtain fluorescent dye-doped polystyrene microspheres is disclosed in Opt Lett. 2013 Apr. 15; 38(8):1197-9 by Lowe K T et al.
A polymerization methodology has also to be adopted which would allow for narrow size distribution of the fluorophore incorporated polymer.
Various polymerization techniques like suspension, emulsion, dispersion etc are utilized to obtain narrow disperse polymer beads in size ranges varying from nanometer to micrometer depending on the size requirement for different applications. Among these methods, dispersion polymerization is a very attractive one for the large-scale preparation of monodisperse polymer beads in the 0.5-15 micrometer size range. The original dispersion polymerization method as developed in the early 1960 did not facilitate the incorporation of fluorophores, functional comonomers, crosslinkers etc. Narayanan, K. et al in Macromol. Sci. Polym. Rev. C41 (1&2), 2001, 79-94 describes preparation of linear and crosslinked polymer microspheres by dispersion polymerization. In presence of “extra” reagents such as fluorophores, functional comonomers, crosslinkers the polymerization would lose control over particle size and especially with crosslinkers result in coagulation of the polymer.
CN101824191 discloses sulfonated polystyrene microspheres coated with a shell layer of p-phenylenevinylene (PPV) fluorescent conjugated macromolecular polymer.
U.S. Pat. No. 5,470,502 discloses a fluorescent pigment having a mean particle size of from 8 to 16 μm, comprising an apolar polymer matrix selected from the group consisting of polymethylmethacrylate, polystyrene.
U.S. Pat. No. 6,165,661 discloses photoconductive imaging members with perylene dimer photogenerating pigment mixtures.
In the early 2000s, M. A. Winnik et al showed that polymer beads with narrow size distribution could be achieved in the dispersion polymerization method by delayed introduction of the crosslinker after the nucleation stage, which they named as the “two-stage” dispersion polymerization (J. Am. Chem. Soc. 2004, 126, 6562-6563). Using this procedure they demonstrated the successful incorporation of up to 3 mol % of the crosslinker divinyl benzene (DVB) into PS and still obtained monodisperse particles.
PBI as well as OPV are chromophores well-known for their tendency to aggregate and lead to quenching of fluorescence in concentrated solution and solid state. Sometimes strategies like introduction of bulky substitutes that can reduce aggregation induced fluorescence quenching or positive effects of aggregate emission based on some J-type aggregates have been reported in literature especially for perylenebisimides to exhibit fluorescence in the solid state. Some bay substituted perylene bisimides have been shown to exhibit intense fluorescence emission in the solid state. The solid state emissions in most of these cases are aggregate emission in the ˜600 nm range.
Although fluorescent microspheres are commercially available, there are certain disadvantages accompanied such as cost of the fluorophore, limited choice of the emission colors, leaking of dyes, high particle size, nonuniformity in dispersion, coagulation of the polymer, hence there is need to design efficient fluorescent microspheres with narrow size distribution and wide range of emission color in solid as well as solution state using easily adaptable procedure.
Therefore, the present inventors incorporated organic fluorophores as crosslinker into commercially available polymer like that of polystyrene (PS), by adopting two-stage dispersion polymerization route to produce highly fluorescent monodisperse PS beads. Among the stable organic fluorophores which have high quantum efficiency, perylene 3,4,9,10-tetracarboxylic diimide (PBI) and oligo(p-phenylenevinylene) (OPV) fluorophores are some of the most preferred due to their strong absorption and fluorescence quantum yield combined with outstanding chemical, thermal and photochemical stability.
PS beads containing both blue emitting OPV and red emitting PBI which were covalently incorporated as cross-linkers in a single polymer based solid state white light (CIE co-ordinates; X=0.33, Y=0.32) as well as multicolor emission with high quantum yield is reported from. The isolated blue emitting OPV and new Orange-red emitting PBI, were covalently incorporated into PS beads as cross-linkers. By changing the feed ratio of both the OPV and PBITEG chromophores we could tune the color in the solid state from blue to white and orange-red. As per the best of our knowledge, this is the first example of solid state white light emission from a single polymer system, where chromophore isolation was used as the strategy to obtain multiple emission from different RGB components.